This invention relates to production from petroleum wells, and in particular, an improvement to the efficiency of plunger lift techniques and an improvement to the efficiency of chemical treatment of wells using plunger lift techniques. The invention will find use in wells which produce either gas or oil, including those using enhanced production techniques such as artificial gas lift. It is anticipated that the invention will find the greatest use in low producing or “stripper” wells, and more particularly in gas wells, thus, the background description and the description of the preferred embodiments of the invention will focus on gas wells.
As a well matures, the pressure in the formation decreases and the volume and velocity of the gas flow slows. While initially the flow rate and velocity of produced gas may be sufficient to remove the liquids with the gas, at some point the flow rate of gas will be insufficient to carry liquids out of the well. As a result, the liquid loading in the well will increase, and liquid will collect in the bottom of the well. When production by natural reservoir pressure becomes uneconomical, artificial lift techniques can be utilized to increase well production. A number of artificial lift systems are known in the industry, including sucker rod pumps, gas lift techniques and plunger lift techniques.
Conventional plunger lift systems, which are also known as free piston systems, utilize a plunger (piston) that is dropped into the well by releasing it from a plunger catcher located at the surface. The plunger falls to the bottom of the tubing and onto a bumper or stop at the bottom of the tubing. After the well is shut in and pressure in the well has built, the wellhead is opened to a low pressure line and the high pressure gas located within the well pushes the piston upward to the surface, thereby pushing the liquid on top of the plunger to the surface. This sequence can be repeated by closing the wellhead off and allowing the plunger to fall again to the bottom of the well while pressure in the well is allowed to rebuild. Another technique is the use of a bypass plunger which is designed so as not to require the well to be shut in. U.S. Pat. No. 6,209,637 entitled “Plunger Lift with Multi Piston and Method” relates to this technique.
Automatic control of plungers used in plunger lift technique is known in the art. Generally, an electronic controller can be utilized which is able to control all of the various valves required to open and close the well, monitor the position of the plunger, and catch the plunger at the surface. Such controllers may, for example use pressure within the well, production flow rate, or travel time of the plunger in order to determine when to perform various operations. Alternatively, an electronic controller may simply operate based on a preset, timed schedule. Electronic controllers are offered by Ferguson Beauregard of Tyler, Tex. and are described at Ferguson Beauregard's web site located at http://www.fergusonbeauregard.com/lift.shtml.
The efficiency of a plunger in bringing fluid to the surface is limited somewhat by the plunger's ability to create a seal with the inside walls of the tubing in which it travels. Ideally, to maximize the seal between the plunger and tubing, a plunger would have a diameter that is as close to the inside diameter of the tubing as possible, thereby minimizing gaps between the plunger and tubing. Unfortunately, obstructions may exist within the tubing, such as sand, crimping from work over operations, tool traps, and the like. Such obstructions could cause the plunger to become stuck in the tubing, thus a smaller diameter plunger may be selected. However, a smaller plunger may tend to leave a gap between the outer surface of the plunger and the inner surface of the production tubing. Thus, it is less able to create an effective seal with the tubing. As a result, when the plunger is rising in the well driving fluids out, these same fluids are able to pass around the sides of the plunger and fall back into the well.
Previous attempts to address this problem have achieved limited success. For example, attempts have been made to use a plunger fitted with a flexible rubber seal able to engage the walls of the tubing. U.S. Pat. No. 7,080,692 titled “Plunger Lift Tool and Method of Using the Same” to Kegin is illustrative of the rubber seal model. However, these plungers suffer from common drawbacks such as insufficient contact with the tubing, wear (particularly in the case of rubber seal plungers), and the inability to accommodate significant aberrations in the tubing. Thus, there is a need for a plunger which is able to travel in a well with a reduced risk of becoming stuck, while still being able to create an effective seal with the tubing walls.
Plunger lift assisted wells are known to be susceptible to corrosion, scale, and undesirable deposits of paraffin, petroleum distillates, asphaltines, microbial growth, and other undesirable substances. To address these problems, treatment chemicals such as soap, acid, corrosion inhibitors, solvents for paraffin and petroleum distillates, stabilizers, biocides and other known treatment chemicals are deposited downhole. A number of techniques have been employed to deliver these treatment chemicals, however, these techniques have many drawbacks, such as excessive chemical use, and inefficient application.
One treatment technique, known as continuous injection, involves the continuous pumping of treatment chemical into the annulus between the tubing and the casing, sometimes through capillary tubes. However, the treatment chemicals themselves are potentially damaging to the production tubing and/or casing, and the use of capillary tubes presents problems associated with installation and maintenance of the tubes themselves. Another treatment technique is a batch treatment technique. However, the batch technique does not provide even distribution of treatment chemical. Many batch and capillary treatment methods rely on the liquids accumulated in the wellbore to dilute the chemicals. As such, the chemicals are generally applied downhole in concentrated form. Unfortunately, the concentrated chemicals can be corrosive to the tubing and casing. Furthermore, in wells where the plunger does not make a good seal with the tubing, fluids which would have been forced out of the well by the plunger lift action will instead flow around the plunger and down the sides of the tubing, washing away treatment chemical, thereby raising the chemical dose necessary for effective treatment. The present invention provides an apparatus and method which is able to evenly distribute treatment chemicals along the inner surface of the tubing and minimize waste of treatment chemicals, damage to the tubing, and disruption to production from the well.
The drawings illustrate certain preferred embodiments of the invention and like elements have been provided with like reference numerals to corresponding items between various drawings.